This invention relates to the production of hollow-core structures, and more particularly to the production of a superplastically formed/diffusion bonded hollow core rotor blade for a gas turbine engine, especially a fan blade, and the procedure for producing such a blade.
Superplasticity is the characteristic demonstrated by certain metals to develop unusually high tensile elongations with minimum necking when deformed within a limited temperature and strain rate range. This characteristic, peculiar to certain metal and metal alloys, has been known in the art as applied to the production of complex shapes. It is further known that at these same superplastic forming temperatures the same materials can be diffusion bonded with the application of pressure at contacting surfaces. One particularly well known process for producing superplastically formed structures, known as the "four sheet process", is described in U.S. Pat. No. 4,217,397, assigned to the McDonnell Douglas Corporation, and herein incorporated by reference.
In a continuing effort to improve gas turbine engine operating efficiencies, as well as to permit the development of transport aircraft having greater passenger and cargo capacities, engine manufactures have been designing increasingly larger engines. These new generation engines, known as "high-bypass engines" or "very high bypass engines", typically operate with a bypass ratio approaching or exceeding 80%, meaning that 80% or more of the total airflow into the engine bypasses the core engine (consisting of the compressor, combustor, and at least the high pressure turbine) and instead flows only through the surrounding fan section, which includes the fan blades and perhaps the low pressure turbine. In a high-bypass engine, most of the generated thrust is derived from the bypass air, enabling higher fuel efficiency and the lower noise output necessary to meet increasingly stringent noise regulations. As a result, engine fan diameters continue to increase in size, and it becomes ever more critical to reduce the structural weight and dynamic loading in the rotating portions of the engine. Estimates are that an effective hollow core fan blade design for a typical large high bypass engine would reduce engine weight by 150 pounds, which would in turn reduce specific fuel consumption by about 5%. As future engine fan diameters increase, it becomes even more critical to reduce the structural weight and dynamic loading in the rotating portions of the engine.
Current fan blade configurations are fabricated from solid titanium materials. This is due to manufacturing cost considerations, as opposed to structural load requirements. Therefore, if a cost effective titanium hollow core fan blade could be manufactured, it would be able to meet the structural load criteria for safe operation. Dynamic loads within the engine core would also be reduced with the reduction in blade mass, which would in turn allow the entire engine core to be further optimized. Future engine growth would occur without requiring the costly redesign of core sections.
It is known in the prior art to manufacture hollow core fan blades for large gas turbine engines by machining matching cavities in titanium plates, then diffusion bonding a honeycomb core inside the cavity. However, this is an extremely expensive process and the resulting blade tends to be vulnerable to damage, in part because it has a discontinuous leading edge. What is needed, therefore, is a hollow core fan blade which may be manufactured by a cost efficient superplastic forming/diffusion bonding (SPF/DB) process, and which is more damage tolerant than currently known hollow core blades.